Parkinson's disease (PD) is a progressive neurological disorder that often results in postural instability, exemplified by increasing loss of balance and incidence of falls. Research has been limited on identifying innovative physical interventions that improve postural balance and lower-extremity muscle strength to maintain daily physical functioning in individuals with PD. Tai Chi, a balance-exercise modality, shown to be effective in improving balance and preventing falls, may be beneficial for individuals with PD;however, little evidence is available concerning the effects of Tai Chi on PD. Based on preliminary work, this application proposes a prospective, 6-month randomized, controlled, double-blind clinical trial to investigate the effects of a Parkinson-specific Tai Chi exercise program in persons with PD. The study will be comprised of three experimental arms: Tai Chi, conventional strength training, and a low-impact exercise control. The primary aim of this investigation is to test the hypothesis that a specially tailored Tai Chi program will improve primary outcome measures of balance and lower-extremity muscle strength, and secondary outcome measures of physical performance, falls self-efficacy, fear of falling, and falls frequency, in persons with mild to moderate PD (stages 1, 2, or 3 of the Hoehn and Yahr scale). Three secondary aims are to examine: (a) whether improvements in the endpoint of balance resulting from the 6-month Tai Chi intervention are associated with reductions in subsequent falls during a 6-month post-intervention period;(b) whether gains in lower-extremity muscle strength following the Tai Chi intervention mediate the relationship between the Tai Chi intervention and the endpoint of balance;and (c) whether gains in balance and muscle strength measures achieved by the Tai Chi group at the end of the trial phase will be sustained during the 6-month post-intervention follow-up phase. Randomized participants (N = 135) in each intervention arm will receive a 60-minute exercise program, 2 times per week, for 26 consecutive weeks. Primary and secondary endpoint outcomes will be collected at baseline, 3 months, and at the 6-month termination, and again at 6 months post-intervention. Data will be analyzed through repeated-measures analysis of covariance on the continuous variables, with the group assignment as a between-group factor, and time to follow-up assessment as a repeated measures factor, using baseline measures as covariates. Based on theoretical considerations and pilot work, the general structure of our hypotheses is that Tai Chi will show better performance results on primary and secondary outcome measures compared to the strength training condition or the low-impact exercise control. This study is unique because it represents an effort to offer a novel exercise intervention to a sample of PD patients who are confronted with postural instability and mobility problems.